1. Field of the Invention
This invention relates to a method for judging the correctness or incorrectness of a prospective contour point, which has been detected as being present on a contour of an irradiation field on a recording medium, in order to recognize where an irradiation field lies on a recording medium in the course of reading out a radiation image which has been recorded on the recording medium such as a stimulable phosphor sheet.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value designed for the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is image-processed and then used when the X-ray image is reproduced as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality and exhibiting such characteristics as high contrast, high sharpness, high graininess or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is used when the radiation image of the object is reproduced as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
A radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using silver halide in that the amount of light emitted by the stimulable phosphor sheet is proportional to the energy intensity of the radiation, to which the stimulable phosphor sheet is exposed when an image is recorded thereon, and the energy intensity of said radiation may be selected from a very wide range (latitude) of radiation energy intensities. If an appropriate read-out gain is selected and used when the light emitted by said stimulable phosphor sheet is being detected, a desirable density can be obtained in the finally reproduced visible image regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed.
In order to detect an image signal accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. A novel radiation image recording and reproducing system which accurately detects an image signal has been proposed in, for example, U.S. Pat. No. 4,527,060. The proposed radiation image recording and reproducing system is constituted such that a preliminary read out operation (hereinafter simply called "preliminary read out") is carried out for approximately ascertaining the radiation image stored on the stimulable phosphor sheet. In the preliminary read out the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary read out is analyzed. Thereafter, a final read out operation (hereinafter simply called "final read out") is carried out for obtaining the image signal, which is to be used during the reproduction of a visible image. In the final read out the stimulable phosphor sheet is scanned sheet with a light beam having an energy level higher than the energy level of the light beam used in the preliminary read out, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out conditions" as used hereinafter means generically various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image read out and the output of a read-out means, i.e. the shape of the image signal. For example, the term "read-out conditions" includes the value of the read-out gain and scale factor which define the relationship between the input to the read-out means and the output therefrom. The term might also include the power of the stimulating rays used for image read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the exposing light beam, i.e. the sensitivity of the phosphor sheet to the exposing light beam depends upon the wavelength of the exposing light beam, the term "energy level of a light beam" means that the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, is weighted with the sensitivity of the phosphor sheet to the wavelength of the exposing light beam. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed with which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary read out is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary read-out image signal) obtained and to adjust an image processing condition, which is used when the image signal is processed, on the basis of the results of the analysis of the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to the systems using stimulable phosphor sheets.
Various methods have been proposed for calculating how the read-out conditions for final read out and/or the image processing condition should be adjusted on the basis of an analysis of the image signal (including the preliminary read-out image signal). As one of such methods, it has been proposed in, for example, U.S. Pat. No. 4,682,028 to create a histogram of the image signal. When a histogram of the image signal is created, the characteristics of a radiation image recorded on a recording medium such as a stimulable phosphor sheet or an X-ray film can be ascertained based on, for example, the maximum value of the image signal, the minimum value of the image signal, or the value of the image signal at which the histogram is maximum, i.e. the value which occurs most frequently. Therefore, if the read-out conditions for the final read out, such as the read-out gain or the scale factor, and/or the image processing condition such as the gradation processing condition or the frequency response processing condition are based on an analysis of the histogram of the image signal, it becomes possible to reproduce a visible image suitable for viewing, particularly for diagnostic purposes.
On the other hand, in the course of radiation image recording, it is often desirable for portions of the object not related to a diagnosis or the like to be prevented from being exposed to radiation. Further, when the object portions not related to a diagnosis or the like are exposed to radiation, the radiation is scattered by such portions to the portion that is related to a diagnosis or the like, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, an irradiation field stop is often used for limiting the irradiation field to an area smaller than the overall recording region of the recording medium so that radiation is irradiated only to that portion of the object which is to be viewed.
However, in cases where the read-out conditions for the final read out and/or an image processing condition are calculated on the basis of the results of an analysis of the image signal in the manner described above and the image signal is detected from a recording medium, on which a radiation image has been recorded by limiting the irradiation field, the radiation image cannot be ascertained accurately if the image signal is analyzed without taking the shape and location of the irradiation field into consideration. As a result, incorrect read-out conditions and/or an incorrect image processing condition is set, so that a visible radiation image suitable for viewing, particularly for diagnostic purposes, cannot be reproduced.
In order to eliminate the aforesaid problem, it is necessary to recognize the shape and location of an irradiation field and then to calculate the read-out conditions for the final read out and/or an image processing condition on the basis of the image signal representing image information stored in the region inside of the irradiation field.
The applicant has proposed various methods for recognizing an irradiation field as disclosed in, for example, U.S. patent application Ser. No. 760,862. The proposed methods allow the aforesaid problem to be eliminated by recognizing where the irradiation field lies on the recording medium, and calculating the readout conditions for the final read out and/or an image processing condition on the basis of only an image signal corresponding to the region thus recognized.
In general, in the disclosed methods for recognizing an irradiation field, several points which are considered to be present on a contour of the irradiation field, i.e. several prospective contour points, are detected Thereafter, the straight lines or curves connecting the prospective contour points are detected, and the region surrounded by the straight lines or curves is recognized as the irradiation field.
A novel method for detecting a prospective contour point has been proposed in, for example, U.S. patent application Ser. No. 760,862. The proposed method comprises the steps of reading out a radiation image which has been recorded on a recording medium in order to obtain an image signal, sampling and digitizing the image signal so that a digital image signal component represents the image information at each position of a predetermined number of positions on the recording medium, and carrying out differentiation processing of the digital image signal components representing image information stored at positions located along a single line on the recording medium. Points at which the absolute value of the differentiated values obtained during the differentiation processing exceed a predetermined threshold value are detected as prospective contour points. In cases where several such points are present, the point nearest to an edge of the recording medium is detected as a prospective contour point. Further, another method of recognizing the irradiation field has been proposed in copending U.S. patent application Ser. No. 182,685. In the proposed method, the irradiation field is recognized by obtaining digital image data for a plurality of positions on the stimulable phosphor sheet from the image signals, detecting prospective edge points, which are considered to be edge portions of the irradiation field on the stimulable phosphor sheet, on the basis of the image data of positions radially outwardly arranged in a plurality of directions from a predetermined point inside the irradiation field, and recognizing as the irradiation field the region surrounded by the lines passing through the prospective edge points. Alternatively, a prospective contour point may be detected by, for example, a method utilizing pattern matching, or a method wherein a straight line is applied and the contour of an irradiation field is discriminated from an inclination of the straight line.
However, in cases where the portion of an image, such as the image of the edge of a bone, at which the image density changes sharply as it also does at the contour of an irradiation field, is present in a radiation image, or energy from scattered radiation has been stored in the region outside of an irradiation field on a recording medium, a prospective contour point is often detected incorrectly. It is difficult to completely eliminate the incorrect detection of a prospective contour point. However, if it were possible to determine that a prospective contour point had been detected incorrectly, said prospective contour point could be canceled, or a correction could be made so that a prospective contour point detected by a different method were employed, instead of said prospective contour point detected incorrectly. In this manner, an irradiation field could be prevented from being recognized incorrectly.